Use of Xpert MTB/RIF Test could Shorten Duration of Airborne Isolation

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Authors (including presenting author) :
Ho MY, Yeung YC, To WK, Kwok HC, Chan YH, Chan MC, Lee WM, Law CB, Chan NY, Tsang Owen
Affiliation :
Department of Medicine and Geriatrics, Princess Margaret Hospital
Introduction :
Patients with suspected pulmonary TB infection will be kept in single-room airborne isolation until proven non-infectious. Conventionally, three negative sputum acid-fast bacilli (AFB) smear results are required to take off isolation. However, the waiting time for collecting 3 separate sputum and laboratory processing time can take up to a week. Xpert MTB/RIF test is a new and simpler test with real-time PCR. It has higher sensitivity and specificity than sputum microscopy, as well as short turn-around time of 2 hours only. Studies have found that use of Xpert MTB/RIF test can shorten duration of isolation thereby decrease costs.
Objectives :
1)To investigate the indication of Xpert MTB/RIF test 2)To see the correlation with sputum AFB smear and culture result
Methodology :
Hospitalized patients with Xpert MTB/RIF test done in the period from January to March, 2018 in PMH were reviewed. Tests are regarded as cost-effective (indicated) if they fulfilled either of the following: 1) Isolation facilities management, especially when the occupancy of isolation bed is high 2) Clinical indication: urgent diagnosis of TB is needed due to use of ventilatory support or high flow oxygen
Result & Outcome :
118 Xpert MTB/RIF tests were sent in this period, involving 112 patients. Majority had 1 test performed. 4 patients had 2 tests and 1 patient had 3 tests performed in same hospitalization due to high clinical suspicion of TB. 96 (81.4%) tests were performed in patients put on airborne isolation, in which 58 (49.2%) cases were taken off isolation once the test became negative. Within these 96 tests, 8 (6.8%) patients had urgent need for early diagnosis. There were 14 (11.9%) positive and 103 (87.3%) negative Xpert tests. One test was invalid and was repeated. Out of the 14 positive tests, 9 had positive sputum AFB smear, while 5 were negative. Three of these five cases were confirmed MTB by culture, showing a higher sensitivity than AFB smear alone. Out of 103 negative tests, there were 5 positive sputum AFB smear results. 3 of them were indeed mycobacterium-other-than-tuberculosis later confirmed by culture, thereby avoiding unnecessary airborne isolation and anti-TB treatment before culture result available. Two had a positive conventional MTB PCR test, both confirmed MTB by culture. Therefore in cases with high clinical suspicion, one single negative Xpert test is not confident enough to exclude infectious TB. Conclusion: Majority (81.4%) of the Xpert MTB/RIF tests performed fulfilled the indications, with two false negative cases only. If good clinical judgement is exercised, additional funding for more frequent use of Xpert MTB/RIF tests can be cost-effective.

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